The proportion of time spent sleeping, and in the various stages of sleep, varies considerably across mammalian species (see Figure 1). A sleep cycle, the period of one episode of short-wave sleep followed by an episode of REM sleep, also varies across species. For laboratory rats, one sleep cycle lasts an average of 10–11 minutes; for humans, one cycle lasts 90–110 minutes. Across species, cycle duration is inversely related to metabolic rate; that is, small animals, which tend to have high metabolic rates (see Chapter 9), have short sleep cycles, and large species have long sleep cycles.

Figure 1  Amounts of Different Sleep States in Various Mammals

There is also a high correlation between total amount of sleep per day and waking metabolic rate: small animals sleep more than large species, at least among plant-eating species (see Figure 2a). Since smaller mammals lose heat faster (see Chapter 9), they burn more energy per gram of body weight just to maintain body temperature, and so perhaps they sleep more to conserve more energy. Interestingly, predatory species show no such correlation, perhaps because meat eaters tend to get more sleep than prey species do (see Figure 2b).

Figure 2  Relationship between Body Size and Sleep Time
(a) Among plant eaters, the larger the body is, the less time is spent asleep. (b) Meat eaters sleep a lot, no matter what their body size—presumably because, as predators rather than prey, they are more secure when asleep. (From Siegel, 2005.)

Except for birds when they’re migrating, all vertebrates appear to show a circadian distribution of activity, a prolonged phase of inactivity, raised thresholds to external stimuli during inactivity, and a characteristic posture during inactivity. Many invertebrates also have clear periods of behavioral inactivity that include heightened arousal thresholds and distinctive postures (B. A. Klein, 2003; Koh et al., 2008)


Klein, B. A. (2003). Signatures of sleep in a paper wasp. Sleep 26: A115–A116.

Koh, K., Joiner, W. J., Wu, M. N., Yue, Z., et al. (2008). Identification of SLEEPLESS, a sleep-promoting factor. Science 321: 372–376.

Siegel, J. M. (2005). Clues to the function of mammalian sleep. Nature 437: 1264–1271.